The present invention relates to an article serving as storage tanks, pipelines, and various equipments associated with them for cryogenic substances, such as liquefied natural gas (LNG), in which all or part of members of the article are formed of a Fexe2x80x94Ni low thermal expansion coefficient alloy and are assembled by welding. (Such an article will be collectively referred to as xe2x80x9cwelded structurexe2x80x9d herein.) The present invention also relates to a welded pipe used in the above pipelines. Furthermore, the present invention relates to a welding material (wire) suitable for use in manufacturing the welded structure and the welded pipe as described above.
Heretofore, austenitic stainless steels, such as JIS-SUS 304, have been used as a material of storage tanks and pipelines for cryogenic substances, such as the liquefied natural gas (LNG). However, the austenitic stainless steel has a high thermal expansion coefficient. Thus, for example in the pipeline, it is required to measure for absorbing the deformation due to expansion and contraction by interposing a loop pipe for each given length of the pipeline. If the pipeline can be formed of a material having a remarkably low thermal expansion coefficient which allows the loop pipe to be eliminated, an elbow pipe involved in the loop pipe will become unnecessary and thereby the diameter of a tunnel for letting the pipeline pass through will be able to be reduced. This will allow maintenance operations for thermal insulations of the pipeline or the like to be minimized, and will open the way to significantly economize construction costs and operation-maintenance expenses.
Among materials available for the transport or storage equipment of cryogenic substances, such as LNG, in view of mechanical and chemical properties thereof, it is known that a Fexe2x80x94Ni alloy of a particular component ratio has an extremely low linear expansion coefficient. Typical examples of such an alloy are Fexe2x80x9436% Ni alloy and Fexe2x80x9442% Ni alloy, which are collectively referred to as Invar alloy (xe2x80x9c%xe2x80x9d concerning each content of components means xe2x80x9cweight %xe2x80x9d herein). These alloys are used as a material for equipment in which the expansion, and/or contraction due to temperature variation, is undesirable.
When a structure formed of the above Fe-Ni low thermal expansion coefficient alloy is assembled by welding, it is desireable to apply a welding material having a linear expansion coefficient similar to that of the base material. Thus, there are proposed some welding materials similar to the base metal, which are disclosed in Japan patent Laid-Open Publication No. 4-231194 and Japanese Patent Laid-Open Publication No. 8-267272 for example.
The welding material disclosed in the above Japanese Patent Laid-Open Publication No. 4-231194 includes C: 0.05 to 0.5% and Nb: 0.5 to 5% as well as Ni (Co) and Fe, and, as needed, selectively includes Mn, Ti, Al, Ca, Mg, Si, Cu, Ag, B, Sn, and Zn, whereby cracking in welding operations can be prevented.
The welding material disclosed in the Japanese Patent Laid-Open Publication No. 8-267272 includes Ni: 30 45%, C:0.03 to 0.3%, Nb: 0.1 to 3%, P:0.015% or less, S:0.005% or less, Si: 0.05 to 0.6%, Mn: 0.05 to 4%, Al: 0.05% or less, and O (oxygen): 0.015% or less, wherein the relationship between Nb and C is defined by (%Nb) xc3x97 (%C) xe2x89xa7 0.01, whereby the reheat cracking in the multi-layer welding operation is prevented and the toughness of the welded zone is also improved.
However, when the above proposed welding materials are applied to assemble a large size structure, particularly to the structure of pipelines, storage tanks or the like for the liquefied natural gas, the following problems arise; various alloying elements, such as Nb, Ti, Ce, Mg, B, Ca et al., included in these welding materials, deteriorate workability of the alloy, resulting in a complicated manufacturing process of a welding material (wire). In particular, Nb contributes to the creation of a large size of oxide, resulting in not only deteriorated hot-workability, but also deteriorated cold-workability. Even if a welding material is successfully produced, the following problems remain.
When welded structures, such as pipelines or storage tanks for liquefied natural gas, are assembled, in view of the efficiency of operations and the quality of welded zones, it is desired that it is capable of automatic welding based on the TIG or plasma welding process and of welding in various positions for on-site operations. That is, the welding material is required to have an excellent weldability in fabrication. Specifically, in automatic welding, it is required not to allow weld defects to arise, such as incomplete formation of root pass bead or lack of fusion cased by insufficient weld penetration or missing of a weld line straightness, and not to allow burn though or lack of penetration in root pass to arise even during the welding in an overhead position.
The proposed welding materials described above were developed under consideration for improving welded joint properties, such as cracking resistance and toughness. However, the aforementioned workability and weldability in fabrication of the welding material were left out of view.
The welding materials disclosed in the Japanese Patent Laid-Open Publication No. 4-231194 were invented under consideration of the solidification cracking in welding operations. However, it does not discuss any measure for the reheat cracking arising in a multi-layer welding operation for a thick member, such as a large size structure. Further, despite of the necessity that the region, which contacts to liquefied natural gas, should have a sufficient toughness under very low temperature, i.e., xe2x88x92196xc2x0 C., this point is also left out of view.
The term xe2x80x9csolidification crackingxe2x80x9d means a cracking which arises in a weld metal (bead) during solidification. The term xe2x80x9creheat crackingxe2x80x9d means a cracking which arises in an initially formed weld metal (bead), which originally had no cracking, by a thermal affection at the time when the initially formed weld metal is reheated by an additional weld metal superposed thereon.
While a measure against the reheat cracking was also considered in the Japanese Patent Laid-Open Publication No. 8-267272, an effect of preventing the reheat cracking is not always sufficiently provided only by arranging the chemical composition of the welding material because a certain dilution of the alloy components due to the welding methods or groove shapes used in welding operations is involved. Thus, it is difficult to actually apply this welding material for structures handling cryogenic substances, such as the liquefied natural gas.
In these structures, such as storage equipment and pipelines for LNG, intended for handling the cryogenic substances, the weld metal is required to have an adequate stress corrosion cracking resistance, as well as toughness under a low temperature, as described above. This is required because the above structures are often coated with a an insulating material (thermal insulator), such as urethane resin, which includes a small amount of Clxe2x80x94, and these structures are often situated close to an ocean and exposed to the atmosphere containing Clxe2x80x94 due to sea water.
The above invention disclosed in the Japanese Patent Laid-Open Publication No. 8-267272 has an objective to improve the cold toughness of welded zone. However, it does not discuss any measure for the stress corrosion cracking resistance.
In the meantime, the welding material used for fabricating the above welded structure is required to be readily produced, or to be readily converted into a wire (i.e. required to have good workability), and is also required to facilitate welding operations when such a material is applied (i.e. required to have good welding weldability in fabrication).
For manufacturing of welding materials, i.e., welding wires, a hot working of raw materials and a cold working for wire drawing are essential. As described above, the welding materials proposed until the present time have an inferior workability, resulting in a complexified process for converting into wires and also an increased manufacturing cost.
In assembling operations for a large size of welded structure, such as storage tanks or pipelines for LNG, an automatic welding based on the TIG or plasma welding process is applied to enhance the efficiency of operations. In addition, it is required to provide an excellent weldability in fabrication, such as a capability of welding in various positions, because such assembling operations are often carried out on site. Specifically, it is required not to allow weld defects to arise, such as the incomplete formation of root pass bead, or lack of fusion caused by insufficient weld penetration, or missing of a weld line in the automatic welding operation, and not to allow the burn through or incomplete formation of root bead to arise during the welding operation in the overhead position. However, such requirements have not been considered in the welding materials proposed until this time.
It is the primary objective of the present invention to provide a welded structure and welded pipe having the following features {circle around (1)} to {circle around (3)};
{circle around (1)} no solidification cracking and no reheat cracking in a weld metal,
{circle around (2)} excellent cold toughness of a weld metal, and
{circle around (3)} 3 excellent stress corrosion cracking resistance of a weld metal.
It is the second objective of the present invention to provide a welding material which is capable of forming a weld metal having the above features {circle around (1)} to {circle around (3)} and which also has the following properties;
{circle around (4)} excellent in_workability, i.e., easily converted into wires, and
{circle around (5)} excellent in weldability in fabrication, i.e., excellent capability of providing a sound welded joint through automatic welding in all positions.
The welded structure of the present invention has members joined to each other by welding, and at least one of the members is formed of a Fexe2x80x94Ni-base low thermal expansion coefficient alloy. The welded structure is characterized by the following (A) and (B);
(A) A weld metal of the welded structure comprises, on the weight % basis, Ni: 30 to 45%, Co: 0 to 10%, C: 0.03 to 0.5%, Mn: 0.7% or less, either one of or the total of Nb and Zr: 0.05 to 4%, and rare earth element: 0 to 0.5%, P as an impurity: 0.02% or less, Al as an impurity: 0.01% or less, and oxygen as an impurity: 0.01% or less.
(B) Each of Si and S in said weld metal satisfying the following formulas {circle around (1)} and {circle around (2)} where each element symbol in the following formulas {circle around (1)} and {circle around (2)} indicates the content (weight %) of each element;
xe2x80x83Sixe2x89xa60.1(Nb+Zr)+0.05%xe2x80x83xe2x80x83{circle around (1)}
Sxe2x89xa60.0015(Nb+Zr)+0.0055%xe2x80x83xe2x80x83{circle around (2)}
The welded pipe of the present invention is produced by shaping a plate of Fexe2x80x94Ni-base low thermal expansion coefficient alloy in tubular shape and then welding its abutting portions. This pipe is used primarily in a pipeline for low temperature cryogenic substances. The pipeline is assembled by circumferential butt welding_of plural pipes. The term xe2x80x9cwelded pipexe2x80x9d also includes a pipe joint, such as a branch tube or an elbow, applied in a particular portion of the pipeline, as long as such a pipe joint is produced by welding.
The welded pipe of the present invention also has said features (A) and (B). Preferably, in the weld metal of the welded structure or welded pipe, a carbide existing in columnar crystal grain boundaries is 0.5 to 50 volume % of the weld metal, wherein Nb and/or Zr in said carbide is 20 weight % or more % of said carbide.
The welding material of the present invention is suitable to be used for forming a weld metal of Fexe2x80x94Ni low thermal expansion coefficient alloy, and is characterized by the following features (C) and (D);
(C) The material is a Fe-base alloy comprising, on the weight % basis, C: 0.5% or less, Ni: 30 to 45%, Co: 0 to 10 %, Mn: 0.7% or less, either one of or the total of Nb and Zr: 0.2 to 4%, and rare earth element(s): 0 to 0.5%, P as an impurity: 0.02% or less, A1:0.01% or less, and oxygen: 0.01% or less.
(D) Each of Si, Nb, Zr, S, C, Mn, O (oxygen) and Al in said welding material satisfies the following formulas {circle around (1)} to {circle around (6)}
Sixe2x89xa60.1(Nb+Zr)+0.05%xe2x80x83xe2x80x83{circle around (1)}
Sxe2x89xa60.0015(Nb+Zr)+0.0055%xe2x80x83xe2x80x83{circle around (2)}
Cxe2x89xa70.015(Nb+Zr)+0.04%xe2x80x83xe2x80x83{circle around (3)}
0.1xe2x89xa6(Si/Mn)xe2x89xa62xe2x80x83xe2x80x83{circle around (4)}
S+Oxe2x89xa60.015%xe2x80x83xe2x80x83{circle around (5)}
Al+Oxe2x89xa60.015%xe2x80x83xe2x80x83{circle around (6)}
where each element symbol in the above formulas {circle around (1)} to {circle around (6)} indicates the content (weight %) of each element.
The inventors have intimately researched factors of cracking in a weld metal, which arises during multi-layer welding in a structure made of Fexe2x80x94Ni low thermal expansion coefficient alloy; cold toughness and stress corrosion cracking resistance of the weld metal; and workability and weldability in fabrication of a welding material. The present invention has been accomplished based on various new knowledge as described below obtained from these researches.
(1) Solidification Cracking and Reheat Cracking of Weld Metal
The inventors have intimately researched the fracture surface of the weld metal, in which solidification cracking and reheat cracking existed. As a result, the following facts have been learned.
(a) The fracture surface of the solidification cracking includes a significant trace of fusion. The fracture surface includes concentrated Si and C thereon.
(b) The fracture surface of the reheat cracking includes, similar to the solidification cracking surface, a potion having concentrated Si and C, and another portion showing a flat intergranular fracture having concentrated S.
Based on these facts, the solidification cracking could be explained as a phenomenon in which the weld metal had a portion where a liquid phase of the concentrated Si and C resided for a long time and then the weld metal was cracked at said portion under a certain external force. It was also believed that the reheat cracking was caused by two factors; Si and C forms a eutectic compound having a low melting point in conjunction with Fe in the matrix, and the resulting compound is heated and molten by subsequent welding passes to cause the cracking; and a fixing force of grain boundaries in the bead is weakened due to the segregation of S at the grain boundaries and this portion is cracked by a thermal stress from subsequent passes.
The inventors confirmed that fixing C as carbide is effective to jointly preventing the two kinds of cracking. If C is crystallized as a stable carbide at a high temperature when the weld metal solidifies, the eutectic compound having a low melting point due to Si and C is not formed during the solidification and thereby the phenomenon concerning the lengthy remanence of the liquid phase may be eliminated. The crystallization of carbide at high temperature also contributes effectively to the creation of the complexified configuration of the columnar crystal grain boundaries during the solidification and to disperse the thermal stress yielded therein so as to reduce stress concentration. Accordingly the cracking can be prevented even if some liquid phase exists in the columnar crystal grain boundaries. The solidification cracking is supposedly prevented by this combined action.
When C in the weld metal exists as a stable carbide, Si, segregated at the columnar crystal grain boundaries, does not create the eutectic compound having a low melting point in conjunction with C and Fe even if a previous bead is reheated by a thermal cycle in multi-layer welding. Further, the crystallization of carbide contributes to the creation of the complexified configuration of the columnar crystal grain boundaries so that the area of the boundary increases. Thus, the amount of the segregation of S is reduced and thereby the deterioration in the fixing force of grain boundaries is limited. Therefore, the cracking due to the concentration of S does not arise. Consequently, the reheat cracking can also be prevented.
Nb and Zr are the most effective elements to form the above-mentioned carbide. Either one of these elements may be effectively applied, or both of them may otherwise be applied together. A suitable content of Nb and/or Zr in the weld metal is required to satisfy the following formulas {circle around (1)} and {circle around (2)};
Sixe2x89xa60.1(Nb+Zr)+0.05%xe2x80x83xe2x80x83{circle around (1)}
Sxe2x89xa60.0015(Nb+Zr)+0.0055%xe2x80x83xe2x80x83{circle around (2)}
(2) Cold Toughness of Weld Metal
As described above, creating the carbide is effective to prevent cracking in welding. However, an excessive amount of the carbide leads to a deteriorated cold toughness. Thus, in order to satisfy a practically sufficient impact value in equipment used under a low temperature, the amount of the carbide in the weld metal is required to be defined up to a predetermined limit.
(3) Improvement of Stress Corrosion Cracking Resistance of Weld Metal
It has been revealed that the stress corrosion cracking of the Fexe2x80x94Ni low thermal expansion coefficient alloy is initially caused by corrosion arising in the grain boundaries having the segregated S. This grain boundary corrosion tends to arise in the coarse columnar crystal grain boundaries of the weld metal that has larger solidification segregation than the mother metal. This corrosion is also effectively prevented by crystallization of carbide in the columnar crystal grain boundaries during the solidification which complexifies the configuration of the columnar crystal grain boundaries so as to reduce the amount of the segregated S in the grain boundaries.
A surface treatment is also effective to prevent stress corrosion cracking. Applying the surface treatment, such as shot peening, sand blasting et al., allows compressive stress to remain on the surface of the weld metal, so that the stress corrosion cracking resistance can be enhanced. A coating may be applied to isolate the weld metal from a corrosive environment. For example, the welded metal surface formed by circulating welding may be coated with a suitable organic substance, such as a combination of an epoxy resin cured by an amine and a polyol resin cured by isocyanate, or one component type epoxy resin cured by ketimine, so as to prevent from stress corrosion cracking.
As described above, the effect of preventing the cracking in the weld metal and improving the stress corrosion cracking resistance of the weld metal can be obtained by providing C in the weld metal as the carbide. This utilization of the carbide is the most important means to solve various problems involved in the welding of the Fexe2x80x94Ni low thermal expansion coefficient alloy. This carbide is required to exist in the columnar crystal grain boundaries of the solid weld metal. Preferably, the amount of the carbide is 0.5 volume % or more of the weld metal, however, since its exceeding existence leads to a deterioration of cold toughness, the amount of the carbide should be limited to 50 volume % or less.
The element needed to form the carbide can be any element, such as Cr, Mo, Ti, Ta, Hf, Nd et al., which forms stable carbide at high temperature. Preferably, at least the carbide of Nb and/or Zr, i.e., (Nb, Zr) C is included. Further, the content of Nb and/or Zr in the carbide is preferable to be 20 weight % or more of the carbide because the carbide can be stable and have a higher melting point in this case.
(4) Workability of Welding Material
As described above, adding Nb and/or Zr is effective in order to improve the properties of the weld metal. However, Nb and Zr tend to create large size oxides because these elements have a strong affinity with oxygen. These oxides dissolve S in the weld metal and contribute to the prevention of the reheat cracking. On the other hand, these oxides increase the deformation resistance during the production of the welding material so that workability is significantly deteriorated.
The inventors have found out that providing Nb and/or Zr as carbide, i.e. (Nb, Zr) C, in the welding material could prevent the creation of the large size oxides so that the workability of the welding material can be improved. The required amount of C in the welding material for creating said carbide increases according to the increase of the amount of Nb and/or Zr. Thus, the contents of C and Nb and/or Zr are required to satisfy the following formula {circle around (3)};
Cxe2x89xa70.015(Nb+Zr)+0.04%xe2x80x83xe2x80x83{circle around (3)}
(5) Improvement of Weldability in Fabrication
It has been revealed that weldability in fabrication is bound up with the contents of Si, Mn, S, O and Al, as the fact described below.
(A) The arrangement of the ratio of respective contents of Si and Mn, i.e., Si/Mn, in the range of 0.1 to 2.0 prevents burn though or drop-down arising during the most difficult welding operation, i.e., the welding in overhead position, and makes it possible to obtain a flat weld melt. Thus, the contents of Si and Mn are required to satisfy the following formula {circle around (4)};
0.1xe2x89xa6(Si/Mn)xe2x89xa62xe2x80x83xe2x80x83{circle around (4)}
(B) The uniformity of a welded bead is bound up with the contents of S and O (oxygen) in the welding material, and the total content of S and O should be 0.015% or less in order to obtain uniform beads of straightness. Thus, the contents of S and O are required to satisfy the following formula {circle around (5)};
S+Oxe2x89xa60.015%xe2x80x83xe2x80x83{circle around (5)}
(C) When the total content of Al and O (oxygen) is greater than 0.015%, a large amount of slag arises, and this makes the heat input from the arc to the base material insufficient, resulting in the lack of penetration in the root pass of the TIG multi-layer welding. Further, a keyhole cannot be sufficiently formed during the plasma welding operation. Thus, the contents of Al and O are required to satisfy the following formula {circle around (6)}
Al+O less than 0.015%xe2x80x83xe2x80x83{circle around (1)}
FIG. 1 is a sectional view showing a groove shape for TIG welding, employed in the example;
FIG. 2 is a sectional view showing a groove shape for plasma welding, employed in the example;
FIG. 3 is a sectional view showing a method to measure the flatness of a welded bead;
FIG. 4 is a sectional view showing a deposition method for TIG welding, employed in the example;
FIG. 5 is a sectional view showing a deposition method for plasma welding, employed in the example;
FIGS. 6a and 6b are sectional views showing an evaluation method of reheat cracking, employed in the example;
FIG. 7 is a sectional view showing a test specimen for a Charpy impact test, employed in the example; and
FIG. 8 is a sectional view showing a test specimen for a stress corrosion cracking test, employed in the example.